1. Field of the Invention
The present invention relates to a semiconductor laser device having a current blocking layer.
2. Description of the Prior Art
A refractive index guided semiconductor laser device supplied with refractive index difference in a direction parallel to an active layer for forming a light guide is developed in general. FIG. 34 is a typical sectional view showing a conventional semiconductor laser device 120 described in Japanese Patent Laying-Open No. 8-222801 (1996).
In the semiconductor laser device 120 shown in FIG. 34, an n-type cladding layer 122, an active layer 123, a p-type cladding layer 124 and a p-type contact layer 127 are successively formed on an n-type substrate 121, and the p-type contact layer 127 and the p-type cladding layer 124 are etched for forming flat portions on a ridge portion and on both sides of the ridge portion.
Further, a first current blocking layer 125 having a low carrier concentration is formed on the flat portions of the p-type cladding layer 124 located on both sides of the ridge portion, and an n-type current blocking layer 126 is formed on the first current blocking layer 125 having a low carrier concentration. A p-type contact layer 128 is formed on the p-type contact layer 127 and the n-type current blocking layer 126.
When the semiconductor laser device 120 is driven, a reverse bias voltage is applied to a p-n junction between the n-type current blocking layer 126 and the p-type cladding layer 124. Thus, the n-type current blocking layer 126 cuts off a current so that the current is injected into the ridge portion in a narrowed state.
In general, a p-n junction formed between an n-type current blocking layer and a p-type cladding layer has large electric capacitance, and hence serves as a factor inhibiting high-speed operation of a semiconductor laser device. The electric capacitance of the p-n junction is increased as the carrier concentration in this p-n junction is increased.
Therefore, the semiconductor laser device 120 shown in FIG. 34 is provided with the current blocking layer 125 having a low carrier concentration, in order to reduce the electric capacitance in the p-n junction between the n-type current blocking layer 126 and the p-type cladding layer 124.
This current blocking layer 125 has a lower carrier concentration than the n-type current blocking layer 126. Therefore, the current blocking layer 125 having a low carrier concentration defines a depletion region in the p-n junction between the n-type current blocking layer 126 and the p-type cladding layer 124, for reducing the electric capacitance. Thus, the semiconductor laser device 120 is enabled to operate at a high frequency.
In the semiconductor laser device 120 having the current blocking layer 125 of a low carrier concentration having a narrower band gap than the p-type cladding layer 124, however, valence bands of the p-type cladding layer 124 and the current blocking layer 125 of a low carrier concentration have energy band structures shown in FIG. 35.
FIG. 35 is a model diagram showing the energy band structures of the valence bands of the p-type cladding layer 124 and the current blocking layer 125 having a low carrier concentration. As shown in FIG. 35, the band gap of the current blocking layer 125 having a low carrier concentration is sufficiently smaller than the band gap of the p-type cladding layer 124, and hence carriers are readily injected from the p-type cladding layer 124 into the current blocking layer 125 having a low carrier concentration and stored therein. Consequently, since depletion of the p-n junction between the n-type current blocking layer 126 and the p-type cladding layer 124 is inhibited, electric capacitance between the current blocking layer 125 having a low carrier concentration and the p-type cladding layer 124 is increased. Therefore, the operating speed of the semiconductor laser device 120 cannot be sufficiently increased.
An object of the present invention is to provide a semiconductor laser device sufficiently increased in operating speed.
A semiconductor laser device according to an aspect of the present invention comprises an active layer, a first cladding layer of a first conduction type provided on the active layer, a current blocking layer of a second conduction type provided on the first cladding layer except a current injection region, a low carrier concentration layer provided on the side of the current blocking layer between the first cladding layer and the current blocking layer and having a lower carrier concentration than the current blocking layer and a depletion enhancement layer provided on the side of the first cladding layer between the first cladding layer and the current blocking layer for inhibiting storage of carriers in the low carrier concentration layer.
In the semiconductor laser device, the depletion enhancement layer inhibits storage of carriers from the first cladding layer into the low carrier concentration layer. Thus, the low carrier concentration layer is kept in a depleted state. Therefore, electric capacitance between the current blocking layer and the first cladding layer is kept small for sufficiently increasing the operating speed of the semiconductor laser device.
The band gaps of the first cladding layer, the depletion enhancement layer and the low carrier concentration layer may be reduced in this order.
Thus, the depletion enhancement layer having an intermediate band gap is provided between the first cladding layer having a large band gap and the low carrier concentration layer having a small band gap.
In this case, the band offset between the first cladding layer and the depletion enhancement layer is smaller than the band offset between the first cladding layer and the low carrier concentration layer, whereby carriers are hardly injected from the first cladding layer into the depletion enhancement layer while carriers are more hardly injected into the low carrier concentration layer. Further, the carriers are injected from the first cladding layer into both of the low carrier concentration layer and the depletion enhancement layer in a divided manner, whereby the quantity of carriers stored in the low carrier concentration layer is reduced. Thus, storage of carriers in the low carrier concentration layer can be inhibited by the simple structure of setting the band gap of the depletion enhancement layer to the intermediate level between the low carrier concentration layer and the first cladding layer.
The first cladding layer may have a flat portion formed on the active layer and a ridge portion formed on the flat portion in the current injection region, the depletion enhancement layer may be formed on the flat portion located on both sides of the ridge portion and on the side surfaces of the ridge portion, and the low carrier concentration layer and the current blocking layer may be successively formed on the depletion enhancement layer.
In this case, the depletion enhancement layer inhibits storage of carriers from the flat portion of the first cladding layer into the low carrier concentration layer. Thus, the low carrier concentration layer is kept in the depleted state, and the electric capacitance between the flat portion of the first cladding layer and the current blocking layer is kept small.
The thickness of the depletion enhancement layer is preferably at least 10 nm. Thus, the semiconductor laser device is more improved in high-frequency characteristic.
The thickness of the depletion enhancement layer is preferably at least 15 nm. Thus, the semiconductor laser device is further improved in high-frequency characteristic.
The semiconductor laser device may further comprise a ridge-shaped second cladding layer of a first conduction type provided on the depletion enhancement layer in the current injection region, the depletion enhancement layer may be formed on the first cladding layer, and the lower carrier concentration layer and the current blocking layer may be successively formed on the depletion enhancement layer located on both sides of the second cladding layer and on the side surfaces of the second cladding layer.
In this case, the depletion enhancement layer inhibits storage of carriers from the first cladding layer into the low carrier concentration layer. Thus, the low carrier concentration layer is kept in the depleted state and the electric capacitance between the first cladding layer and the current blocking layer is kept small.
The thickness of the depletion enhancement layer is preferably at least 15 nm. Thus, the semiconductor laser device is more improved in high-frequency characteristic.
The thickness of the depletion enhancement layer is preferably at least 20 nm. Thus, the semiconductor laser device is further improved in high-frequency characteristic.
The depletion enhancement layer, the low carrier concentration layer and the current blocking layer may be successively formed on the first cladding layer except the current injection region, and the semiconductor laser device may further comprise a second cladding layer of a first conduction type provided to fill up a space enclosed with the side surfaces of the depletion enhancement layer, the low carrier concentration layer and the current blocking layer and the upper surface of the first cladding layer in the current injection region.
In this case, the depletion enhancement layer inhibits storage of carriers from the first cladding layer into the low carrier concentration layer. Thus, the low carrier concentration layer is kept in the depleted state and the electric capacitance between the first cladding layer and the current blocking layer is kept small.
The thickness of the depletion enhancement layer is preferably at least 15 nm. Thus, the semiconductor laser device is more improved in high-frequency characteristic.
The thickness of the depletion enhancement layer is preferably at least 20 nm. Thus, the semiconductor laser device is further improved in high-frequency characteristic.
The depletion enhancement layer may have a single-layer structure or a superlattice structure.
The active layer may include a layer made of (Alx1Ga1xe2x88x92x1)y1In1xe2x88x92y1P, the depletion enhancement layer may be made of (Alx2Ga1xe2x88x92x2)y2In1xe2x88x92y2P or Alx2Ga1xe2x88x92x2As, the low carrier concentration layer may be made of (Alx3Ga1xe2x88x92x3)y3In1xe2x88x92y3P or Alx3Ga1xe2x88x92x3As, the current blocking layer may be made of (Alx4Ga1xe2x88x92x4)y4In1xe2x88x92y4P or Alx4Ga1xe2x88x92x4As, and x1, x2, x3, x4, y1, y2, y3 and y4 may be at least zero and not more than 1 respectively.
The active layer may include a layer made of Alx1Ga1xe2x88x92x1As, the depletion enhancement layer may be made of Alx2Ga1xe2x88x92x2As, the low carrier concentration layer may be made of Alx3Ga1xe2x88x92x3As, the current blocking layer may be made of Alx4Ga1xe2x88x92x4As, and x1, x2, x3 and x4 may be at least zero and not more than 1 respectively.
The active layer may be made of Inx1Ga1xe2x88x92x1N, the depletion enhancement layer may be made of Alx2Ga1xe2x88x92x2N, the low carrier concentration layer may be made of Alx3Ga1xe2x88x92x3N, the current blocking layer may be made of Alx4Ga1xe2x88x92x4N, and x1, x2, x3 and x4 may be at least zero and not more than 1 respectively.
The active layer preferably includes a layer made of (Alx1Ga1xe2x88x92x1)y1In1xe2x88x92y1P, the depletion enhancement layer is preferably made of (Alx2Ga1xe2x88x92x2)y2In1xe2x88x92y2P, the low carrier concentration layer is preferably made of Alx3Ga1xe2x88x92x3As, the current blocking layer is preferably made of Alx4Ga1xe2x88x92x4As, x1, x2, x3, x4, y1 and y2 are preferably at least zero and not more than 1 respectively, and the first conduction type is preferably the p type, and the second conduction type is preferably the n type.
In this case, improvement of the high-frequency characteristic resulting from the depletion enhancement layer inhibiting storage of carriers from the first cladding layer into the low carrier concentration layer is particularly remarkable.
A semiconductor laser device according to another aspect of the present invention comprises an active layer, a first cladding layer of a first conduction type provided on the active layer, a first current blocking layer having a low carrier concentration provided on the first cladding layer except a current injection region and a depletion enhancement layer formed between the first cladding layer and the first current blocking layer for inhibiting storage of carriers in the first current blocking layer, while the depletion enhancement layer has an energy level in band gap supplying second conduction type carriers to compensate for first conduction type carriers supplied from the first cladding layer due to a modulation doping effect.
The first current blocking layer having a low carrier concentration is an undoped layer or a layer doped with a low density of impurity In a range capable of blocking a current.
In the semiconductor laser device, the depletion enhancement layer formed with the energy level in band gap supplying the second conduction type carriers is formed between the first cladding layer and the first current blocking layer.
In this case, the second conduction type carriers supplied from the energy level in band gap of the depletion enhancement layer compensate for the first conduction type carriers supplied from the first cladding layer. Therefore, storage of carriers can be prevented in the first current blocking layer having a low carrier concentration. Thus, the first current blocking layer is kept in a depleted state. Therefore, electric capacitance generated between the first current blocking layer and the first cladding layer can be reduced and the operating speed of the semiconductor laser device can be sufficiently increased.
At this point, the first current blocking layer has a narrower band gap than the first cladding layer. When the first current blocking layer has a narrower band gap than the first cladding layer, carriers are readily injected from the first cladding layer into the first current blocking layer and stored therein. In this case, however, the depletion enhancement layer formed between the first cladding layer and the first current blocking layer can inhibit storage of carriers in the first current blocking layer.
The energy level in band gap preferably has such density that substantially all energy level in band gap ionize under a condition applying no bias voltage voltage. In this case, it is possible to effectively compensate for the first conduction type carriers supplied from the first cladding layer. Therefore, storage of carriers in the first current blocking layer having a low carrier concentration can be more effectively inhibited.
The energy level in band gap may be formed by doping with a second conduction type impurity. In this case, the depletion enhancement layer provided with the energy level in band gap can be readily formed.
The material of the depletion enhancement layer may be the same as the material of the first current blocking layer. In this case, the band gap width of the depletion enhancement layer and the first current blocking layer are equalized with each other.
The first cladding layer may have a larger band gap than the depletion enhancement layer, and the semiconductor laser device may further comprise an intermediate band gap layer provided between the first cladding layer and the depletion enhancement layer and having a band gap smaller than the band gap of the first cladding layer and larger than the band gap of the depletion enhancement layer.
In this case, carriers are hardly injected from the first cladding layer into the depletion enhancement layer and hardly injected into the first current blocking layer having a low carrier concentration either due to the intermediate band gap layer provided between the first cladding layer and the depletion enhancement layer. In this case, further, the carriers are injected into both of the depletion enhancement layer and the intermediate band gap layer in a divided manner, and hence hardly injected into the first current blocking layer.
Thus, storage of carriers in the first current blocking layer is further inhibited.
Further, the ranges of the thickness and the carrier concentration of the depletion enhancement layer capable increasing the operating speed of the semiconductor layer device are widened by providing the intermediate band gap layer in the aforementioned manner. Therefore, the thickness and the carrier concentration of the depletion enhancement layer can be readily set so that the depletion enhancement layer can be readily prepared.
The depletion enhancement layer may have a band gap smaller than the band gap of the first cladding layer and larger than the band gap of the first current blocking layer. In this case, the depletion enhancement layer serves as the aforementioned intermediate band gap layer, thereby further inhibiting storage of carriers in the first current blocking layer.
Also in this case, the ranges of the thickness and the carrier concentration of the depletion enhancement layer capable of increasing the operating speed of the semiconductor laser device are widened. Thus, the thickness and the carrier concentration of the depletion enhancement layer can be readily set so that the depletion enhancement layer can be readily prepared.
The first cladding layer may have a flat portion formed on the active layer and a ridge portion formed on the flat portion in the current injection region, the depletion enhancement layer may be formed on the flat portion located on both sides of the ridge portion and on the side surfaces of the ridge portion, and the first current blocking layer may be formed on the depletion enhancement layer. In this case, a ridge guided semiconductor laser device improved in operating speed is implemented.
The depletion enhancement layer and the first current blocking layer may be successively formed on the first cladding layer except the current injection region, and the semiconductor laser device may further comprise a second cladding layer of a first conduction type provided to fill up a space enclosed with the side surfaces of the depletion enhancement layer and the first current blocking layer and the upper surface of the first cladding layer in the current injection region. In this case, a self-aligned semiconductor laser device improved in operating speed is implemented.
The depletion enhancement layer may be formed on a region excluding the current injection region. In this case, a current is quickly injected into the current injection region provided with no depletion enhancement layer of the opposite conduction type.
The semiconductor laser device may further comprise a second current blocking layer of a second conduction type provided on the first current blocking layer.
A semiconductor laser device according to another aspect of the present invention comprises an active layer, a first cladding layer of a first conduction type provided on the active layer, a first current blocking layer having a low carrier concentration provided on the first cladding layer except a current injection region and a depletion enhancement layer formed between the first cladding layer and the first current blocking layer for inhibiting storage of carriers in the first current blocking layer.
In the semiconductor laser device, the depletion enhancement layer inhibits storage of carriers from the first cladding layer into the first current blocking layer having a low carrier concentration. Thus, the first current blocking layer having a low carrier concentration is kept in a depleted state. Therefore, electric capacitance between the first current blocking layer having a low carrier concentration and the first cladding layer is kept small for sufficiently increasing the operating speed of the semiconductor laser device.
At this point, the first current blocking layer having a low concentration has a narrower band gap than the first cladding layer. When the first current blocking layer having a low carrier concentration has a narrower band gap than the first cladding layer, carriers are readily injected from the first cladding layer into the first current blocking layer having a low carrier concentration and stored therein. In this case, however, the depletion enhancement layer formed between the first cladding layer and the first current blocking layer having a low carrier concentration can inhibit storage of carriers in the first current blocking layer having a low carrier concentration.
The band gaps of the first cladding layer, the depletion enhancement layer and the first current blocking layer having a low carrier concentration may be reduced in this order.
Thus, the depletion enhancement layer having an intermediate band gap is provided between the first cladding layer having a large band gap and the first current blocking layer having a low carrier concentration having a small band gap.
In this case, the band offset between the first cladding layer and the depletion enhancement layer is smaller than the band offset between the first cladding layer and the first current blocking layer having a low carrier concentration, whereby carriers are hardly injected from the first cladding layer into the depletion enhancement layer while carriers are more hardly injected into the first current blocking layer having a low carrier concentration. Further, the carriers are injected from the first cladding layer into both of the first current blocking layer having a low carrier concentration and the depletion enhancement layer in a divided manner, whereby the quantity of carriers stored in the low carrier concentration layer is reduced. Thus, storage of carriers in the first current blocking layer having a low carrier concentration can be inhibited by the simple structure of setting the band gap of the depletion enhancement layer to the intermediate level between the first current blocking layer having a low carrier concentration and the first cladding layer.
The first cladding layer may have a flat portion formed on the active layer and a ridge portion formed on the flat portion in the current injection region, the depletion enhancement layer may be formed on the flat portion located on both sides of the ridge portion and on the side surfaces of the ridge portion, and the first current blocking layer having a low carrier concentration may be formed on the depletion enhancement layer.
In this case, the depletion enhancement layer inhibits storage of carriers from the flat portion of the first cladding layer into the first current blocking layer having a low carrier concentration. Thus, the first current blocking layer having a low carrier concentration is kept in the depleted state, and the electric capacitance between the flat portion of the first cladding layer and the first current blocking layer having a low carrier concentration is kept small.
The semiconductor laser device may further comprise a ridge-shaped second cladding layer of a first conduction type provided on the depletion enhancement layer in the current injection region, the depletion enhancement layer may be formed on the first cladding layer, and the first current blocking layer having a lower carrier concentration may be formed on the depletion enhancement layer located on both sides of the second cladding layer and on the side surfaces of the second cladding layer.
In this case, the depletion enhancement layer inhibits storage of carriers from the first cladding layer into the first current blocking layer having a low carrier concentration layer. Thus, the first current blocking layer having a low carrier concentration is kept in the depleted state and the electric capacitance between the first cladding layer and the first current blocking layer having a low carrier concentration is kept small.
The depletion enhancement layer and the first current blocking layer having a low carrier concentration may be successively formed on the first cladding layer except the current injection region, and the semiconductor laser device may further comprise a second cladding layer of a first conduction type provided to fill up a space enclosed with the side surfaces of the depletion enhancement layer and the first current blocking layer having a low carrier concentration layer and the upper surface of the first cladding layer in the current injection region.
In this case, the depletion enhancement layer inhibits storage of carriers from the first cladding layer into the first current blocking layer having a low carrier concentration. Thus, the first current blocking layer having a low carrier concentration is kept in the depleted state and the electric capacitance between the first cladding layer and the first current blocking layer having a low concentration is kept small.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.